1. Field of the Invention
The present invention relates to plating bath analysis methods. More particularly, the present invention relates to a method for monitoring the status of a plating bath purification treatment cycle.
2. Description of Related Art
To ensure optimal performance of a plating bath, the bath must be purified at regular intervals. The purification serves to remove organic contaminants and other impurities which can build up in the bath over time and lead to undesirable plating characteristics. In many cases, the contaminants are breakdown products resulting from the electrochemical processes within the bath. The time between purification treatments varies depending upon the contaminant buildup rate, which is a function of the nature and frequency of the plating bath processes. For example, plating baths using organic addition agents such as the acid copper plating bath generate significant levels of contaminants and require frequent purification.
Carbon treatment is one widely used plating bath purification technique. The carbon treatment process involves contacting activated carbon with the plating bath. The contaminants are adsorbed on the activated carbon and thereby effectively removed from the plating bath solution. The level of contaminants within the bath is continually reduced during the purification treatment, and when an acceptably low contaminant level is attained, the treatment can be terminated. Fresh addition agents are added to the treated bath and the plating bath is ready for use again. On continued usage organics and other contaminants will again begin to accumulate within the bath, and the purification treatment will eventually have to be repeated. Since the quality of the plating bath depends upon maintaining a low level of contaminants, it is essential that the carbon treatment cycles be initiated and terminated at the appropriate times. The same concerns apply to other plating bath purification techniques.
Under current practice, the progress of the purification treatment is typically monitored by repeated manual tests on the plating bath solution. One such manual test, disclosed in LeaRonal Application Note No. AN30009CT, uses a Hull cell plating technique to monitor carbon treatment. During the carbon treatment process, an operator will run a Hull cell plating test approximately once every three hours to determine when the process is complete. Each Hull cell test is performed at 2 amperes for 15 minutes with air agitation. When the plated Hull cell panels appear flat-matte across the entire current density range, the carbon treatment process is considered sufficiently complete. Other methods currently used involve similar repeated manual testing procedures.
The current purification treatment monitoring methods suffer from a number of problems. Repeated performance of manual tests is time-consuming and requires skilled personnel and specialized equipment. The time required to perform the tests translates into production downtime, thereby limiting plating process productivity.
In addition, presently used techniques do not usually lead to accurate and repeatable results. The manual tests often involve guess work and arbitrary interpretation, as in the case of the plated Hull cell panels. This can lead to incomplete purification treatments and resultant plating bath quality problems.
Furthermore, present techniques are not easily integrated with known voltammetric plating bath analysis methods, such as those disclosed in U.S. Pat. No. 4,631,116, and assigned to the present common assignee. Plating bath users must therefore maintain one system and set of equipment for measuring plating bath constituent concentration, and another for tracking the progress of purification treatments.
As is apparent from the above, there presently is a need for a simple and efficient method of continuously monitoring the status of a purification treatment cycle within a plating bath. The method should provide a highly accurate indication of the optimal treatment initiation and termination points. The method should provide these features and also be compatible with most on-line plating bath analysis methods and the equipment associated therewith.